1. Field of the Invention
The present invention relates to a wafer supporting boat used in a vertical-type heat treatment apparatus, more particularly, the present invention relates to a wafer supporting boat used in a vertical-type heat apparatus such as a CVD reactor and a diffusion furnace, which conducts a predetermined heat treatment with rotation thereof in the furnace tube (hereinafter referred to as vertical wafer supporting boat).
2. Related Prior Art
A wafer supporting boat--used in a heat treatment furnace for semiconductor wafers--has had in general a structure that shaft-like or long rectangular members extend parallel to the axis of the furnace tube and which are fixedly spaced by a frame. A plurality of grooves for holding the wafers, are carved or notched at a constant interval on the internal side of each member extending in the axial direction of the furnace tube. The frame (which may be integrally formed with the members) positions the members, extending in the axial direction of the furnace tube, in the crosssectional shape of an arc, which curves following the profile of the sectional internal wall surface of the furnace tube, and a plurality of semiconductor wafers are respectively held in the regularly spaced grooves for holding wafer (hereinafter referred to as wafer holding grooves or simply holding grooves) and regularly disposed in the treatment space of the furnace by a single supporting member or a plurality of supporting members of each wafer holding boat.
As to heat treatment apparatuses in which such a wafer supporting boat is installed, two types of apparatus are generally available. Specifically, a horizontal-type heat treatment apparatus, where the axial line of the furnace tube extends in a horizontal direction and a vertical-type heat treatment apparatus, where the axial line of the furnace extends in a vertical direction are both available. Recently, the vertical-type heat treatment apparatus has been used more generally due to reduction of the required installment floor space and facilitation of operational automation thereof.
A vertical wafer supporting boat has basically horizontal rectangular slit grooves, which are designed to hold wafers in horizontal, or oblique, rectangular slit grooves, which are so designed that the wafers are posed obliquely with each free end upwardly positioned, instead of adopting such groove shapes as V grooves or Y grooves as in the case of a horizontal-type heat treatment apparatus. With such slit grooves as mentioned above, in which wafers have been inserted, upper and lower clearances between a wafer and the inner walls or sides of the holding grooves are small and shortage of reactant material or turbulence of a reaction gas therefore occurs locally in the holding grooves. Such shortage or turbulence causes poor heat treatment or poor external appearance in the portions of the wafers inside the holding grooves and in the neighborhood of a supporting member or members, in which the holding grooves are carved.
In order to prevent these faults, it is considered advantageous to broaden the width of each of the grooves. With broadened grooves, the wafers are, however, subject to wafer chipping and the like due to vibration during heat treatment or other processes.
For the purpose of solving the aforementioned faults and facilitating guidance of wafers, a technology has been disclosed that, as shown in FIG. 4, almost trapezoidal, profile holding grooves 101 are formed, being vertically arrayed, in a side surface of supporting poles 43, 44 to hold the wafers 10 in a linear contact. Although the aforementioned wafer supporting boat usually rotates during heat treatment, the boat can not avoid the faults to any great degree and only provides a small improvement at each end of the openings of the grooves 101 with a reaction gas penetrating from a side, even though each holding groove has a sectional shape which increases in width toward the center of a treatment space.
On the other hand, in such a structure that the edge of each of the wafers contact in a linear fashion with a holding groove, a contact mark between the wafer and the boat (hereinafter referred to as boat contact mark or simply boat mark) or chipping at the edge occurs due to the difference between the thermal expansion coefficients between the wafer and boat and the like. The boat contact mark not only affects adversely the quality of the treated wafer, but also the chips can to be held fast in the form of protrusions in a holding groove or cause particle generation.